Gotowa bibliografia na temat „Inbreeding load”

The authors evaluated the relevant literature related to purging, which is the interaction between selection and inbreeding in which the population may eliminate its inbreeding load at least partially. According to the relevant literature, the inbreeding load and the process of purging were evaluated via pedigree methods based on ancestral inbreeding, the inbreeding–purging model, and expressed opportunity of purging, along with genomic methods. Most ancestral inbreeding-related studies were performed in zoos, where only a small proportion of the studied populations show signs of purging. The inbreeding–purging model was developed with Drosophila, and it was used to evaluate different zoo ungulates and Pannon white rabbits. Purging was detected in both studies. The expressed opportunity of purging was applied in Jersey cattle and Pannon white rabbits. In the Jersey cattle, it had an effect of 12.6% for fitness, while in the Pannon white rabbits, the inbreeding load was between 40% and 80% of its original value. The genomic studies also signalled purging, but they also made it clear that, contrary to the detected purging, the evaluated populations still suffered from inbreeding depression. Therefore, especially for domesticated animals, it can be concluded that deliberate inbreeding with the purpose of generating purging is not advocated.

The magnitude of inbreeding depression in Drosophila melanogaster appears too large to be accounted for by mutational load with multiplicative fitness interactions among loci, if current estimates of mutation and selection parameters are valid. One possible explanation for this discrepancy is synergistic epistasis among the fitness effects of deleterious mutations. A simple model of the effect of synergistic epistasis on the inbreeding load is developed. This model is used to show that deleterious mutations could account for the Drosophila data on the effects of inbreeding on components of fitness such as viability.

We studied the effects of population size on the inbreeding depression and genetic load caused by deleterious mutations at a single locus. Analysis shows how the inbreeding depression decreases as population size becomes smaller and/or the rate of inbreeding increases. This pattern contrasts with that for the load, which increases as population size becomes smaller but decreases as inbreeding rate goes up. The depression and load both approach asymptotic limits when the population size becomes very large or very small. Numerical results show that the transition between the small and the large population regimes is quite rapid, and occurs largely over a range of population sizes that vary by a factor of 10. The effects of drift on inbreeding depression may bias some estimates of the genomic rate of deleterious mutation. These effects could also be important in the evolution of breeding systems in hermaphroditic organisms and in the conservation of endangered populations.

In theoretical biology, a prevailing hypothesis posits a profound interconnection between effective population size (Ne), genetic diversity, inbreeding, and genetic load. The domestication and improvement processes are believed to be pivotal in diminishing genetic diversity while elevating levels of inbreeding and increasing genetic load. In this study, we performed a whole genome analysis to quantity genetic diversity, inbreeding, and genetic load across seven wild Ovis species and five domesticated sheep breeds. Our research demonstrates that the genetic load and diversity of species in the genus Ovis have no discernible impact on recent Ne, and three species within the subgenus Pachyceros tend to carry a higher genetic load and lower genetic diversity patterns. The results coincide with these species’ dramatic decline in population sizes within the subgenus Pachyceros ~80–250 thousand years ago. European mouflon presented with the lowest Ne, lower genetic diversity, and higher individual inbreeding coefficient but a lower genetic load (missense and LoF). This suggests that the small Ne of European mouflon could reduce harmful mutations compared to other species within the genus Ovis. We showed lower genetic diversity in domesticated sheep than in Asiatic mouflon, but counterintuitive patterns of genetic load, i.e., lower weak genetic load (missense mutation) and no significant difference in strong genetic load (LoF mutation) between domestic sheep and Asiatic mouflon. These findings reveal that the “cost of domestication” during domestication and improvement processes reduced genetic diversity and purified weak genetic load more efficiently than wild species.

Abstract The rate of decline in reproductive fitness in populations of Drosophila melanogaster inbred at an initial rate of approximately 1% per generation has been investigated under both competitive and noncompetitive conditions. Breeding population size was variable in the inbred lines with an estimated harmonic mean of 66.7 +/- 2.2. Of the 60 lines maintained without reserves, 75% survived a period of 210 generations of slow inbreeding and were then rapidly inbred by full-sib mating to near-homozygosity. The initial rate of inbreeding was estimated to be 0.96 +/- 0.16% per generation, corresponding to an effective population size of approximately 50. However, the rate of inbreeding declined significantly with time to average only 0.52 +/- 0.08% per generation over the 210 generation period, most likely due to associative overdominance built up by genetic sampling and selection in the small populations. The total inbreeding depression in fitness was estimated to be 87 +/- 3% for competitive ability and 27 +/- 5% for fitness under uncrowded conditions, corresponding to rates of decline of 2.0 +/- 0.3 and 0.32 +/- 0.07%, respectively, per 1% increase in the inbreeding coefficient. The frequency of lethal second chromosomes in the resultant near-homozygous lines was of the order of 5%, lethal free second chromosomes showed a mean viability under both crowded and uncrowded conditions of approximately 95%, and their population cage fitness was 60% that of Cy/+ heterozygotes. It can be concluded that homozygous genotypes from which deleterious genes of major effect have been eliminated during slow inbreeding may show far less depression in reproductive fitness than suggested by earlier studies of wild chromosome homozygotes. The loss in fitness due to homozygosity throughout the entire genome may be as little as 85-90% under competitive conditions, and 25-30% in an optimal environment.

AbstractInbreeding depression is well documented in insects but the degree to which inbreeding depression varies among populations within species, and among traits within populations, is poorly studied in insects other than Drosophila. Inbreeding depression was examined in two long-term laboratory colonies of the seed beetle, Callosobruchus maculatus (Fabricius), which are used frequently as models for experiments in ecology, evolution and behaviour. Inbreeding depression in these laboratory colonies are compared with one recently field-collected population of a different seed beetle, Stator limbatus Horn. Inbreeding reduced embryogenesis, egg hatch and larval survival in both species, such that eggs produced by sib matings were >17% less likely to produce an adult offspring. Inbred larvae also took 4–6% longer to develop to emergence in both species. Inbreeding depression varied among the measured traits but did not differ between the two populations of C. maculatus for any trait, despite the large geographic distance between source populations (western Africa vs. southern India). Inbreeding depression was similar in magnitude between C. maculatus and S. limbatus. This study demonstrates that these laboratory populations of C. maculatus harbour substantial genetic loads, similar to the genetic load of populations of S. limbatus recently collected from the field.

Abstract The goal of this study is to provide information on the genetics of inbreeding depression in a primarily outcrossing population of Mimulus guttatus. Previous studies of this population indicate that there is tremendous inbreeding depression for nearly every fitness component and that almost all of this inbreeding depression is due to mildly deleterious alleles rather than recessive lethals or steriles. In this article I assayed the homozygous and heterozygous fitnesses of 184 highly inbred lines extracted from a natural population. Natural selection during the five generations of selfing involved in line formation essentially eliminated major deleterious alleles but was ineffective in purging alleles with minor fitness effects and did not appreciably diminish overall levels of inbreeding depression. Estimates of the average degree of dominance of these mildly deleterious alleles, obtained from the regression of heterozygous fitness on the sum of parental homozygous fitness, indicate that the detrimental alleles are partially recessive for most fitness traits, with h~∼0.15 for cumulative measures of fitness. The inbreeding load, B, for total fitness is ~1.0 in this experiment. These results are consistent with the hypothesis that spontaneous mildly deleterious mutations occur at a rate >0.1 mutation per genome per generation.

SummaryInbreeding depression has important implications for a wide range of biological phenomena, such as inbreeding avoidance, the evolution and maintenance of sexual systems and extinction rates of small populations. Previous investigations have asked how inbreeding depression evolves in single and subdivided populations through the fixation of deleterious mutations as a result of drift, as well as through the expression of deleterious mutations segregating in a population. These studies have focused on the effects of mutation and selection at single loci, or at unlinked loci. Here, we used simulations to investigate the evolution of genetic load and inbreeding depression due to multiple partially linked loci in metapopulations. Our results indicate that the effect of linkage depends largely on the kinds of deleterious alleles involved. For weakly deleterious and partially recessive mutations, the speed of mutation accumulation at segregating loci in a random-mating subdivided population of a given structure tends to be retarded by increased recombination between adjacent loci – although the highest numbers of fixation of slightly recessive mutant alleles were for low but finite recombination rates. Although linkage had a relatively minor effect on the evolution of metapopulations unless very low values of recombination were assumed, close linkage between adjacent loci tended to enhance population structure and population turnover. Finally, within-deme inbreeding depression, between-deme inbreeding depression and heterosis generally increased with decreased recombination rates. Moreover, increased selfing reduced the effective amount of recombination, and hence the effects of tight linkage on metapopulation genetic structure were decreased with increasing selfing. In contrast, linkage had little effect on the fate of lethal and highly recessive alleles. We compare our simulation results with predictions made by models that ignore the complexities of recombination.

Cette thèse explore des méthodes basées sur la généalogie pour partitionner le gain génétique et le fardeau génétique (FG) dans les races ovines laitières françaises : Lacaune (LAC), Basco-Béarnaise (BB), Manech Tête Noire (MTN) et Manech Tête Rousse (MTR). Le Chapitre 2 a utilisé une analyse rétrospective pour affiner la partition de la tendance génétique dans les échantillonnages mendéliens par catégorie d'animaux définies par le sexe et par la voie de sélection, ainsi que pour caractériser les contributions génétiques à long terme. Nous avons analysé le gain génétique pour la production laitière dans quatre races : LAC, BB, MTN et MTR. Les mères à béliers (MAB) et les mâles d’insémination artificielle (IA) ont été les sources les plus importantes de progrès génétique, comme l'a montré la décomposition des tendances de l'échantillonnage mendélien. Les contributions annuelles étaient plus variables pour les mâles d'IA que pour les MAB, étant donné que ces contributions ont été calculées en moyenne sur un plus petit nombre d'individus. En termes d'échantillonnage mendélien, les femelles ont contribué davantage que les mâles au gain génétique total, et nous interprétons cela comme étant dû au fait les femelles constituent un plus grand réservoir de diversité génétique. En outre, nous avons calculé les contributions à long terme de chaque individu aux pseudo-générations suivantes. L'échantillonnage mendélien était plus important que la moyenne des parents pour déterminer la sélection des individus et leurs contributions à long terme. Ces contributions étaient plus significatives pour les mâles d'IA (dont la descendance est plus importante que celle des femelles) et en BB qu’en LAC (étant une race de taille plus importante).Au Chapitre 3, la théorie qui montre la nature additive du FG est présentée. L'effet du FG et l'effet génétique additif (dans une population non consanguine) ont une corrélation négative dépendant de la fréquence des allèles, de la consanguinité et de la dominance. Nous avons calculé et décrit les coefficients de consanguinité partielle dans trois races : BB, MTN et MTR. Ensuite, nous avons inclus ces coefficients dans un modèle mixte en tant que covariables de régression aléatoire pour estimer la variance et les valeurs génétiques du FG pour la production laitière. Il existe une variance génétique pour le FG dans les races MTN et MTR, mais elle n'était pas différente de zéro pour BB. Comme attendu, nous avons estimé des corrélations génétiques négatives entre le FG et les valeurs génétiques estimées ; cependant, elles étaient proches de zéro dans les trois races. La faible magnitude du FG ne justifie pas une sélection fondée sur ce critère.Dans le Chapitre 4, nous avons évalué l'efficacité de l'intégration du FG dans les stratégies de sélection chez les ovins laitiers. Nous avons simulé 10 générations de sélection. Six scénarios qui diffèrent par les critères de sélection (uniquement les valeurs génétiques additive estimées du caractère, uniquement les valeurs génétiques estimées du FG, ou à la fois les deux) et les stratégies d’accouplement (minimiser le FG ou la consanguinité attendue dans la descendance) ont été évalués. Les scénarios ont été comparés en termes de gain génétique, coefficients et taux de consanguinité, taille efficace et précision de la sélection. Il est possible d'utiliser les prédictions des effets du FG pour sélectionner les animaux directement ou dans le cadre de stratégies d'accouplement. Cependant, la sélection basée sur le FG (en raison de sa variation et de sa magnitude) ne présente pas d'intérêt pratique. À la lumière de nos résultats, l'inclusion d'animaux génotypés pourrait améliorer la précision de la prédiction des FG individuelles. D'autres recherches sont nécessaires
This thesis explores pedigree-based methods to partition genetic gain and inbreeding load in French dairy sheep breeds: Lacaune (LAC), Basco-Béarnaise (BB), Manech Tête Noire (MTN) and Manech Tête Rousse (MTR).The Chapter 2 used a retrospective analysis to fine partitioning genetic trend in Mendelian samplings by categories of animals defined by sex and by selection pathways, and to similarly characterize long-term genetic contributions. We analysed genetic gain for milk yield in four dairy sheep breeds: LAC, BB, MTN and MTR. Dams of males and Artificial Insemination (AI) males were the most important sources of genetic progress as observed in the decomposition in Mendelian sampling trends. The yearly contributions were more erratic for AI males than for dams of males as they are averaged across a smaller number of individuals. Overall, in terms of Mendelian sampling, females contributed more than males to the total genetic gain, and we interpret that this is because females constitute a larger pool of genetic diversity. In addition, we computed long-term contributions from each individual to the following pseudo-generations. Mendelian sampling was more important than Parent Average to determine the selection of individuals and their long-term contributions. Long-term contributions were larger for AI males (with larger progeny sizes than females) and in BB than in LAC (with the latter being a larger population).In Chapter 3, we presented theory that show the additive nature of the inbreeding load. The inbreeding load effect and the regular (in non-inbred population) additive genetic effect have a negative correlation depending on allele frequencies, inbreeding and dominance. We calculated and described the partial inbreeding coefficients in three French dairy sheep populations: BB, MTN and MTR. Then, we included these coefficients in a mixed model as random regression covariates, to predict genetic variance and breeding values of the inbreeding load for milk yield in the same breeds. There is genetic variance for inbreeding load in MTN and MTR breeds, but it was not different from zero for BB. As expected, we estimated negative genetic correlations between inbreeding load and breeding values; however, estimates were close to zero in the three sheep breeds. The small magnitude of inbreeding load does not warrant selection based on this criterion.In Chapter 4, we evaluated the effectiveness of involving inbreeding load in selection strategies in a dairy sheep breeding scheme. We did this by simulation of 10 generations of evaluations and selection. Six scenarios that differ in the criteria of selection (only breeding values, only breeding values of inbreeding load, or both genetic and inbreeding load breeding values) and mate allocation strategies (minimising inbreeding load or minimising expected future inbreeding) were evaluated. Scenarios were compared in terms of genetic gain, inbreeding coefficients, rate of inbreeding, effective population size, and accuracy of selection. The use of predictions of inbreeding load effects to select animals directly or in mating strategies is feasible. However, selection based on inbreeding load (due to its variation and magnitude) is not of practical interest. In light of our results, the inclusion of genotype animals could improve the accuracy of predicting individual inbreeding loads. Further research is needed

The chapter looks into conservation genetics as a method used to mitigate extinction. It explains how molecular genetics is used in conservation biology. Molecular systemics are great assets in wildlife forensic investigations. PCR technology enables genetic studies to be conducted on rare species. The chapter notes careful evaluation is needed as neutral estimates of genetic diversity sometimes differ from measures of adaptive variation. Next, the chapter explores the basis and impact of inbreeding and captive breeding in relation to the genetic load of smaller populations. The chapter expounds on the concept of plant and animal conservations as well. De-extinction, meanwhile, is regarded as only a movie concept.

This chapter looks into ex situ conservation. It acknowledges that in situ conservation techniques might be needed if a successful ex situ conservation programme, which is termed reintroduction, is achieved. The chapter explores the subdivisions of ex situ conservation: intensive or general captive breeding programmes and gene banks. Additionally, it discusses the example of the Lord Howe Island Group's ex situ conservation action on the Lord Howe Island stick insect. The chapter notes the significance of collection, transport, captive breeding, reintroduction, supplementation, and reinforcement in relation to ex situ conservation. It talks about the concept of captive breeding by explaining husbandry, inbreeding, and hybridization. Finally, the chapter includes an interview with applied ecologist Dr. Tim Bray.